Food Biochemistry and Food Processing

8 Enzyme Engineering and Technology 185

COENZYMES, PROSTHETICGROUPS, AND
METALIONCOFACTORS

Nonprotein groups can also be used by enzymes to
affect catalysis. These groups, called cofactors, can
be organic or inorganic and are divided into three
classes: coenzymes, prosthetic groups, and metal
ion cofactors (McCormick 1975). Prosthetic groups
are tightly bound to an enzyme through covalent
bonding. Coenzymes bind to an enzyme reversibly
and associate and dissociate from the enzyme during
each catalytic cycle; therefore, they may be consid-
ered as cosubstrates. An enzyme containing a cofac-
tor or prosthetic group is termed as holoenzyme.
Coenzymes can be broadly classified into three main
groups: coenzymes that transfer groups onto sub-
strate, coenzymes that accept and donate electrons,
and compounds that activate substrates (Table 8.2).
Metal ions such as Ca^2 , Mg^2 , Zn^2 , Mn^2 , Fe^2 ,
and Cu^2 may in some cases act as cofactors. These
may be bound to the enzyme by simple coordination
with electron-donating atoms of side chains (imida-

zole of His, -SH group of Cys, carboxylate O-of

Asp and Glu). In some cases, metals such as Mg^2

are associated with the substrate rather than the

enzyme. For example Mg-ATP is the true substrate

for kinases (Anderson et al. 1979). In other cases,

metals may form part of a prosthetic group in which

they are bound by coordinated bonds (e.g., heme,

Table 8.2) in addition to side-chain groups. Usually

in this case, metal ions participate in electron trans-

fer reactions.

KINETICS OFENZYME-CATALYZEDREACTIONS

The term enzyme kinetics implies a study of the

velocity of an enzyme-catalyzed reaction and of the

various factors that may affect this (Moss 1988). An

extensive discussion of enzyme kinetics would stray

too far from the central theme of this chapter, but

some general aspects will be briefly considered.

The concepts underlying the analysis of enzyme

kinetics continue to provide significant information

Figure 8.7.The substrate binding site of maize glutathione S-transferase. The binding residues are depicted as
sticks, whereas the substrate is depicted in a space fill model. Only Ser 11 is involved directly in catalysis and is
considered as catalytic residue.